Add Tilt integration

[briandominick]

I don't know why we're limited to using Liquid templates in parse actions, since the committers at Tilt have done the work of making a universal translator for templating engines, including Liquid, Haml, Handlebars, Slim, and ERB. Let's let user's declare a templating engine, pass the work on to Tilt, and call it a day.

- action: parse
  data: mydata.json
  builds:
    - template: _templates/haml/left-nav.html
      engine: haml
      output: _build/left-nav.html
    - template: _template/slim/top-nav.html
      engine: slim
      output: _build/top-nav.html